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Research Journal of Animal, Veterinary and Fishery Sciences ___________________________ ISSN 2320 – 6535
Vol. 1(2), 12-15, March (2013)
Res. J. Animal, Veterinary & Fishery Sci.
Review Paper
Operating Procedures for Efficient Anaerobic Digester Operation
Suryawanshi P.C.1, Chaudhari A.B.2*, Bhardwaj S.1 and Yeole T.Y.1
1
Jain R & D, Jain Irrigation Systems Ltd, Jalgaon 425 001, INDIA
School of Life Sciences, North Maharashtra University, Jalgaon 425 001, INDIA
2
Available online at: www.isca.in
Received 23rd January 2013, revised 8th February 2013, accepted 15th February 2013
Abstract
Ignorance or unknown fear of handling anaerobic digester should be removed first for its successful operation. While the
biochemical reactions inside the digester are complex, concurrent and if not regulated, they become counterproductive
resulting into souring or shut down of the digester. For this purpose, detailed procedure is given by focusing underlying
principals which regulate biogas output. Finally, factors affecting digester performance are summarize so that maximum
output of biogas could be generated, leaving little BOD/COD in the effluent to be used as organic manure without defying
the norms set by State Pollution Control Board. For the sake of completeness, method of ORP estimation is given, since its
narrow and critical range determines success or failure of anaerobic digestion.
Keywords: Biogas, methane, anaerobic digestion, solid waste, organic loading rate,
Introduction
Installation and operating the biogas plants is not new to India.
In fact, Khadi and Village Industries Commission (KVIC),
amongst its various mandates, were assigned a program for
recycling rural agri-waste for the production of biogas,
commonly known as gobar gas. Accordingly, gobar gas plants
were installed in millions to generate gobar gas for meeting
energy needs of rural population. However, it is on record that
hardly 0.35 million plants are operational, while the rest have
become defunct. Its critical analysis has revealed that either
right operating procedure was not in place or personnel manning
these plants were not sufficiently trained to operate them
efficiently. The theme of this article precisely focuses on this
aspect.
Starting up the digester
Substrate: The choice of a substrate depends on availability, its
cost, ease of handling, safety for operators, storage and
requirements for feeding. It induces hydrolysis of particulate
matter initially to organic acids and finally to volatile fatty acids
(VFAs).
Inoculum: Seeding of the digester warmed up to 35°C with
fresh cattle dung/manure may be useful as it contains a
consortium of microbes to undertake activities from hydrolysis,
acidogenesis and acetogenesis to methanogenesis. Alternatively,
an inoculum comprising of secondary sludge: primary sludge
(1:10) may be used.
While, secondary sludge is highly concentrated with facultative
anaerobes, primary sludge contains facultative anaerobes and
methane-forming anaerobes. Therefore, it is obvious that
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anaerobic digester cannot be successfully seeded with secondary
sludge alone. Once an anaerobic digester has started operating
efficiently, it can be fed with secondary sludge alone. During
start-up or when the efficiency of the digester deteriorates,
requisite amount of fresh cattle manure is added daily, until
successful start-up and improved efficiency is obtained.
Loading: During start-up, loading to the digester should
proceed slowly. While doing so careful monitoring and control
of pH as well as alkalinity are essential until the digester attains
pH in 6.8-7.2 range. For this purpose, different alkalis could be
added to maintain desired pH, alkalinity and buffering capacity
to neutralize acids within the digester.
Tips for pH and alkalinity maintenance: When the digester
pH is 7.2 or lower, NH4+ is favored. When the digester pH is
greater than 7.2, NH3 is favored. Ammonia-nitrogen
concentration beyond 1500-3000 mg/liter is not only inhibitory;
it creates an additional problem of foam and scum generation.
Digester stability: Approximately one month is required to
achieve a steady-state digester. Once the digester is stabilized,
monitor VFAs: alkalinity ratio.
Sludge feeding: Sludge feeding or organic loading rate (OLR)
is expressed in terms of volatile solids (VS). Typically, it is 0.5–
0.6 kg VS/m3/day and recommended loading is 3.2–7.2 kg
VS/m3/day.
Thickening of sludge is an important operational factor affecting
the digester performance. Raw sludge or feed sludge, which has
3–6 % solids, reduces destruction of volatile solids as well as
methane production. Digested sludge and supernatant are
withdrawn from the digester on a routine basis to be used as
liquid organic manure.
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Research Journal of Animal, Veterinary and Fishery Sciences _______________________________________ ISSN 2320 – 6535
Vol. 1(2), 12-15, March (2013)
Res. J. Animal, Veterinary & Fishery Sci.
Common operational problems: Associated with anaerobic
digesters, these are over-pumping of raw sludge and excessive
withdrawal of the digested sludge. To avoid/minimize these
problems, two significant retention times: (i) solid retention
time (SRT) and (ii) hydraulic retention time (HRT) need to be
considered.
SRT is usually greater than 12 weeks. High SRT values are
advantageous as they maximize sludge removal capacity, reduce
requisite digester volume and provide buffering capacity for
shock loadings and toxic compounds in wastewaters and
sludges. Biological acclimation to toxic compounds permits
significant reduction of BOD/COD minimizes fluctuations in
response to toxicants and maximizes biogas production. These
activities are catalyzed by the following categories of bacteria
summarized in table-1.
Table-1
Approximate generation time of important groups of
wastewater bacteria
Bacterial
Generation
Function
group
time
Floc
formation
and
degradation
of
soluble
Aerobic
organics in the activated 15–30 min
organotrophs
sludge and trickling filter
processes
Hydrolysis and degradation
of organics in the anaerobic
Facultative
digester,
besides
floc
and anaerobic
formation and degradation of 15–30 min
organotrophs
soluble organics in the
activated
sludge
and
trickling filter processes,
Oxidation of NH4+ and NO2Nitrifying
in the activated sludge and
2–3 days
bacteria
trickling filter processes
Sulfate
reducing
Sulfate is reduced to H2S
3-8 hrs
bacteria
MethaneProduction of methane in the
forming
3–30 days
anaerobic digester
bacteria
The conversion of volatile solids (VS) to biogas in an anaerobic
digester is controlled by HRT. It determines the rate and extent
of methane production. Increase in retention time >12 days does
not significantly contribute to increased destruction of VS. The
rate of digestion of sludge varies from season to season, being a
function of temperature. The rate of anaerobic digestion of
sludge and methane production is proportional to digester
temperature.
It has been noted in the above procedure that the following five
parameters determine process configuration of the digesters.
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Process configuration
Batch or continuous: The batch system is simplest, where the
waste is added to the digester at the beginning and sealed for the
duration of the process. Typically, biogas production forms a
normal distribution pattern. Therefore, the operator can
determine when the process of digestion has been completed.
However, it suffers from severe odor problem during emptying
cycles1-5.
In the continuous process, which is common, waste is added in
either stages or continuously, the end products are removed
periodically or continuously, resulting in a continuous
production of biogas. Upflow anaerobic sludge blanket (UASB),
Expanded granular sludge bed (EGSB) and Internal circulation
(IC) digesters belong to this category.
Temperature: Recurring problems of maintenance of
optimum/uniform temperature of the digester needs priority
attention. Secondly, inadequate mixing creates localized pockets
of variation in temperature. In case, the digester in run using
thermophilic anaerobes, they adversely affect digester
performance due to low bacterial growth, high endogenous
death rate of other bacteria and consequently lack of microbial
diversity usually witnessed in the mesophilic digesters.
Therefore, relatively high residual values of VFAs are
registered.
Solid contents: Typically, the digesters are designed to operate
at either high solid content [total suspended solids (TSS) greater
than 20 %] or low solid content (where TSS is less than 15 %).
For high solid digesters, land requirement is less due to dense
slurry and net lower volumes. It also requires more energy input
to process the feedstock.
On the contrary, low solid digesters transport wastes using
pumps that require significantly lower energy input, attain
thorough circulation of waste matter and its intimate contact
with bacterial population, providing higher output efficiency.
However, they require larger land area than required by high
solid digesters due to increase in volumes1,6-9.
Complexity: A single stage (one phase) digester is one in which
all
biochemical
reactions
(hydrolysis,
acidogenesis,
acetogenesis and methanogenesis) occur within its sealed
environment. While it offers benefit of lower construction cost,
it commands less control on the reactions occurring within the
digester. Since microbes involved in these reactions being
different, their optimal pH being either acidic or alkaline and
their physiological needs for better performance differ
significantly, single-stage digesters invariably perform below
optimal level, resulting into either less biogas production or less
% methane in it.
To address the limitation of single stage digester, while
substrate hydrolysis and acidogenesis could be carried out in
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Research Journal of Animal, Veterinary and Fishery Sciences _______________________________________ ISSN 2320 – 6535
Vol. 1(2), 12-15, March (2013)
Res. J. Animal, Veterinary & Fishery Sci.
one digester, acetogenesis and methanogenesis in another
digester. This theme of separation of biochemical processes and
their catalyzing microbial populations has given rise to the
concept of two-stage digester. In a two-stage digester, digestion
vessels are different and their environment is optimized to
promote maximum output from their bacterial communities.
Typically, hydrolysis and acidogenesis occur in the first
digester. Acidogenic bacteria multiply rapidly than
methanogenic bacteria and hence produce organic acids more
quickly. Acetogenesis could be spread over both digesters. The
digested material is then heated to required operational
temperature (either mesophilic or thermophilic) prior to being
pumped into a methanogenic digester. Methanogenic bacteria
require optimum pH 7.2-8.0 and slightly higher temperature in
order to optimize their performance. Thus, separation of
conditions to suit the needs of respective microbial populations
enables two-stage digesters to give superior performance as
judged from higher output of biogas and its enhanced %
methane.
Nutrients: There are significant differences in nutrient
requirements between aerobes and anaerobes. Differences in
critical nutrients emanate due to unique enzyme systems needed
by methane-forming bacteria. In the conversion of acetate to
methane, cobalt (Co), iron (Fe), nickel (Ni), sulfur (S), selenium
(Se), tungsten (W) and molybdenum (Mo) are required.
Additional micronutrients are barium (Ba), calcium (Ca),
magnesium (Mg) and sodium (Na). Macronutrient requirements
for anaerobic processes are much lower than the requirements
for aerobic processes due to lower cell yield.
Nitrogen and phosphorus are made available to anaerobic
processes as ammonical- nitrogen (NH4+) and orthophosphate
(HPO4–). Their amount needed to satisfy acceptable digester
performance can be determined, considering adequate residual
concentrations of soluble nutrients in the digester effluent.
Residual values of 5 mg/liter of NH4+ and 1–2 mg/liter of HPO4–
are usually recommended. Absence of residual nutrients means
that nutrients must be added. While for nitrogen addition,
ammonium chloride, aqueous ammonia and urea may be used,
for phosphorus addition, phosphate salts and phosphoric acid
may be used. While some methanogens are able to fix N2, some
use the amino acid, alanine.
Nutrient requirements for anaerobic digesters vary as a function
of OLR. Generally, COD:N:P of 1000:7:1 is used for high
strength wastes and 350:7:1 for low strength wastes,
respectively. The C/N value of at least 25:1 is suggested for
optimal gas production. Nitrogen is approximately 12% and
phosphorus 2 % of the dry weight of bacterial cells. Both N and
P should not be limited in the digester.
Retention time (RT): If the waste is primarily soluble, short
retention time (RT) is adequate for maximum treatment
efficiency. However, solubilization of particulates being a
relatively slow biochemical action10 for a waste with high
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suspended solids, anaerobic fermentation by longer RT with
aerobic microbial consortium permits optimal solubilization and
subsequent methanogenesis accomplished under anaerobic
conditions. Thus, through the use of innovative, high-rate
reactor designs, anaerobic treatment can now challenge the cost
of aerobic treatment for many soluble wastewater treatment
applications.
If solid retention time (SRT) is short, size of methane-forming
bacteria is reduced.
RT of a substrate in the digester is a function of i. its chemical
nature, ii. one-stage or two-stage digestion, iii. amount and type
of feedstock and iv. physiology of microbial communities
(mesophiles/thermophiles).
In a single-stage mesophilic digester, RT could be 15-30 days or
more depending upon % solid content, while in thermophilic
digester; RT may be in the region of 10-14 days. In the plugflow system, full degradation of the waste may not be achieved
in this time-scale, due to which, the digestate is dark in colour
and has intense odor, a reflection of high BOD/COD level.
In a two-stage mesophilic digester, RT could be 15-30 days,
with ample scope for its reduction under thermophilic
conditions. In mesophilic UASB digesters, HRT may vary from
1-24 hours and solid retention times up to 90 days. In this
manner, UASB system is able to separate solid on the basis of
HRT with the utilization of a sludge blanket.
Continuous digesters have mechanical or hydraulic devices,
depending on the level of solids, to mix them with the
consortium of bacteria to allow excess material to be
continuously digested by maintaining reasonably constant
volume of input as well as output from the digesters2.
Mass transfer consideration: Mass transfer into biomass
occurs rapidly when i. bio-particle surface: volume ratio is high,
ii. bio-particle diameter/bio-film thickness is less and iii. liquid
phase.
After reviewing above factors which determined digester
performance, it is equally necessary to find out the causes which
lead to change in operational conditions and its efficiency.
Causes for changes in operational conditions in
digesters
Of the total microbial population, approximately 80 % are
facultative anaerobes, producing a variety of acids, alcohols,
CO2 and H2 from carbohydrates, lipids and proteins. Facultative
anaerobes (oxygen- tolerant species) and obligate anaerobes
(oxygen – intolerant species) change operational conditions,
which in turn result in changes in i. dominant bacteria and ii.
concentration of acids/alcohols.
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Research Journal of Animal, Veterinary and Fishery Sciences _______________________________________ ISSN 2320 – 6535
Vol. 1(2), 12-15, March (2013)
Res. J. Animal, Veterinary & Fishery Sci.
Since methanogenesis is catalyzed efficiently in defined range
of oxidation-reduction potential (ORP), it is necessary to
familiarize with method of ORP estimation and factors
deviating it from optimum.
References
1.
Dannis A. and Burke P.E., Dairy waste anaerobic digestion
handbook: Option for recovering beneficial products from
dairy manure, Environment Energy Company, Olympia,
WA, USA, 16-20 (2001)
2.
Kim H.W., Han S.K. and Shin H.S., The optimization of
food waste addition as a co-substrate in anaerobic digestion
of sewage sludge, Waste Manag. Res., 21, 515-526 (2003)
3.
Finstein M.S., Zadik Y., Marshall A.T. and Brody D., The
arrow-bio process for mixed municipal solid waste –
responses to requests for information, In: Proc. for
biodegradable and residual waste management, Eds.,
Papadimitriou E.K. and Stentiford E.I., Technology and
Service Providers Forum, 407-413 (2004)
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Bouallagui H., Torrijos M., Godon J.J., Moletta R., Cheikh
R.B., Touhami Y., Delgenes P.P. and Hamdi M., Twophase anaerobic digestion of fruit and vegetable wastes:
bioreactors performance, Biochem. Engg. J., 21, 193-197
(2004)
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Davidsson A., Jansen J.C., Gruvberger C. and Hallmer M.,
Anaerobic digestion potential of urban organic waste: a
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(2007)
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Tchobanoglous G., Burton F.L. and Stensel H.D.,
Wastewater engineering: Treatment and reuse, 4th Edn.,
Metcalf and Eddy, Inc. Toronto, 07-23 (2003)
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Gerardi M.H., The microbiology of anaerobic digesters –
Wastewater microbiology series, Wiley-Interscience, New
York, USA (2003)
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Pandya M.T., Biotechnology applications in the treatment
of industrial wastewater. Water Wastewater Asia, 48-51
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Parkin G.F. and Owen W.F., Fundamentals of anaerobic
digestion of wastewater sludges, J. Environ. Engg. Div.
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Method of ORP estimation
ORP of wastewater or sludge can be obtained by using a pHmeter, with a milli-volt scale and an ORP probe. ORP is an
indicator of the capacity of molecules in wastewater or sludge to
release or gain electrons (oxidation or reduction, respectively).
At values i. less than -100 mV, mixed acids and alcohol
formation may occur, ii. less than -200 mV, methane production
may start and iii. less than -300mV, growth of methane-forming
bacteria does not occur.
Conclusion
Over the years concerted efforts have evolved practical
guidelines which could be metamorphosed into an operating
procedure to run the biogas plant efficiently. Critical analysis of
operational biogas plants has revealed that either right operating
procedure was not in place or personnel manning these plants
were not sufficiently trained to operate them efficiently.
Unawareness or unidentified fear of handling anaerobic digester
should be removed first for its successful operation. While the
biochemical reactions inside the digester are complex,
concurrent and if not regulated, they become counterproductive
resulting into souring or shut down of the digester. For this
purpose, detailed procedure is given by focusing underlying
principals which regulate biogas output. Nutrient requirements
for anaerobic digesters vary as a function of OLR. Finally,
factors affecting digester performance are summarize so that
maximum productivity of biogas could be produced, leaving
slight BOD/COD in the effluent to be used as organic manure
without defying the standards set by State Pollution Control
Board. For the sake of totality, method of ORP estimation is
given, since its narrow and critical range determines success or
failure of anaerobic digestion.
Acknowledgement
Authors thank to Dr. B. H. Jain, Founder Chairman, Jain
Irrigation Systems Limited for providing state-of-the-art R & D
facilities to undertake the present work.
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10. Bansal A.K., Mitra A., Arora R.P., Gupta T. and Singhvi B.
S.M., Biological treatment of domestic wastewater for
aquaculture, J. Agri. Biol. Sci., 2, 6-12 (2007)
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